We present a method for producing a single circularly polarized attosecond pulse by an intense few cycle elliptically polarized laser pulse combined with a terahertz field from numerical solutions of the time-dependent Schr\"odinger equation for the molecular ion ${\mathrm{H}}_{2}^{+}$. It is found that in the presence of a 62.5 THz ($$\lambda${}=4800\text{ }\text{ }\mathrm{nm}$) field at an intensity of $$\sim${}{10}^{14}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$, a single circularly polarized 114 as pulse can be generated by an elliptical polarized laser pulse at a wavelength of 400 nm with an ellipticity of $ϵ=0.59$. The efficiency of circular polarization attosecond pulse generation is interpreted based on a classical model of single electron recollision with the parent ion.

Time-resolved photoelectron holography from atoms using midinfrared laser pulses is investigated by solving the corresponding time-dependent Schr\"odinger equation (TDSE) and a classical model, respectively. The numerical simulation of the photoelectron angular distribution of Xe irradiated with a low-frequency free-electron laser source agrees well with the experimental results. Different types of subcycle interferometric structures are predicted by the classical model. Furthermore with the TDSE model it is demonstrated that the holographic pattern is sensitive to the shape of the atomic orbitals. This is a step toward imaging by means of photoelectron holography.

Three alternative forms of harmonic spectra, based on the dipole moment, dipole velocity, and dipole acceleration, are compared by a numerical solution of the Schrödinger equation for a hydrogen atom interacting with a linearly polarized laser pulse, whose electric field is given by $E\left(t\right)=E_{0}f\left(t\right)\cos ({\omega }_{0}t+\eta )$ with Gaussian carrier envelope $f\left(t\right)=\exp (-t^2∕{\delta }^2)$. The carrier frequency ${\omega }_0$ is fixed to correspond to a wavelength of $800\mathrm{nm}$. Spectra for a selection of pulses, for which the intensity $I_0=c{\epsilon }_0{E}_0^2$, duration $T\propto \delta$, and carrier-envelope phase $\eta$ are systematically varied, show that, depending on $\eta$, all three forms are in good agreement for “weak” pulses with $I_0<I_b$, the over-barrier ionization threshold, but that marked differences among the three appear as the pulse becomes shorter and stronger $(I_0>I_b)$. Except for scalings by powers of the harmonic frequency, the three forms differ from one another only by “limit contributions” proportional to the expectation values of the dipole moment $⟨z\left(t_f\right)⟩$ or dipole velocity $⟨\dot{z}\left(t_f\right)⟩$ at the end $\left(t_f\right)$ of the pulse. For long, weak pulses the limit contributions are negligible, whereas for short, strong ones they are not. In the short, strong limit, where $⟨\dot{z}\left(t_f\right)⟩\ne 0$ and therefore $⟨z\left(t\right)⟩$ may increase without bound (i.e., the atom may ionize), depending on $\eta$, an “infinite-time” spectrum based on the acceleration form provides a convenient computational pathway to the corresponding infinite-time dipole-velocity spectrum, which is related directly to the experimentally measured “harmonic photon number spectrum” (HPNS). For short, intense pulses the HPNS is quite sensitive to $\eta$ and exhibits not only the usual odd harmonics but also even ones. The analysis also reveals that most of the harmonic photons are emitted during the passage of the pulse. Because of the divergence of $⟨z\left(t\right)⟩$ the dipole-moment form does not provide a numerically reliable route to the harmonic spectrum for very short (few-cycle), very intense laser pulses.

We theoretically investigate molecular photoelectron momentum distributions (MPMDs) by bichromatic [frequencies $({\omega }_1,{\omega }_2)]$ circularly polarized attosecond UV laser pulses. Simulations performed on aligned single-electron ${\mathrm{H}}_2{}^{+}$ by numerically solving the corresponding three-dimensional time-dependent Schrödinger equation within a static nucleus frame show that MPMDs exhibit a spiral structure for both co-rotating and counter-rotating schemes. Results are analyzed by attosecond perturbation ionization models. Coherent electron wave packets created, respectively, by the two color pulses in the continuum interfere with each other. Photoionization distributions are functions of the photoelectron momentum $p$ and the ejection angle $\theta$, thus leading to spiral MPMDs. The dependence of spiral MPMDs on the time delay between the bicircular pulses and their relative phases is also presented. The spiral interference patterns are determined by the helicities and frequencies $\left({\omega }_1,{\omega }_2\right)$ of the bicircular fields. It is also found that the spiral patterns are sensitive to the molecular alignment and suppressed by two-center ionization interference, thus offering new tools for imaging molecular geometry.

We demonstrate theoretically an efficient field-free orientation in LiH and LiCl driven by available terahertz few-cycle pulses (TFCPs). Exact results by numerically solving the time-dependent Schrödinger equation including the vibrational and rotational degrees of freedom are compared to the rigid-rotor approximation (RRA) as well as to the impulsive approximation (IA), and the effect of rotational-vibrational coupling on the both RRA and IA is examined in detail. We find that the current available TFCPs may overcome the technical limitation of terahertz half-cycle pulse for enhancing the field-free molecular orientation.